Low-loss, controllable parameter, transmission line



Nov. 26, 1968 QMAMPBELL 3,413,575

LOW-LOSS, CONTROLLABLE PARAMETER, TRANSMISSION'LINE Filed Nov. 10, 19642 Sheets-Sheet 2 INVENTOR, oo-- v. cmpazu 9 M ATTORNEY-S United StatesPatent 3,413,575 LOW-LOSS, CONTROLLABLE PARAMETER,

TRANSMISSION LINE Donn V. Campbell, Neptune, N.J., assignor to theUnited States of America as represented by the Secretary of the ArmyFiled Nov. 10, 1964, Ser. No. 410,324 8 Claims. (Cl. 333-31) ABSTRACT OFTHE DISCLOSURE A constant impedance low-loss delay line which includes acoaxial transmission line section comprising an inner conductor and anouter conductor, and a radially stratified propagation mediumintermediate the conductors which is adapted to be slideably positionedover the inner conductor. The propagation medium includes a plurality offerrite-ceramic elements spaced apart by means of plastic spacers. Eachof the ferrite-ceramic elements comprises a ceramic body and a ferritebody concentrically disposed with respect to the coaxial line conductorsand disposed along a common radial plane. The ferrite-ceramic elementsand plastic spacers therebetween include a radial slot and are alignedto form an integrated longitudinal slot along the inner conductor.

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

This invention relates to electromagnetic wave transmission lines andmore particularly to an improved low loss, controllable parameter,transmission line for use as an RF delay line.

As is well known, it is often desirable to control the characteristicimpedance and electrical length of RF transmission lines. For example, aradio-frequency transmission line with adjustable electrical length isdesirable for introducing a predetermined delay in the feedlines feedingan antenna array. Also, a radio-frequency transmission line with anadjustable characteristic impedance may find application in broadbandimpedance matching of antennas and in adjusting the voltage and currentamplitude in the antenna feed system.

One low-loss continuously variable delay line is described in aco-pending Brueckmann application Ser. No. 315,093 which issued asPatent No. 3,219,950 on Nov. 23, 1965. It capitalizes on the fact thatthe characteristic impedance of a transmission line Whose conductors aresurrounded by air is not changed if the air is replaced by a ferriteWhose effective relative permeability ,u equals the effective relativedielectric constant or permit: tivity e However, the propagationvelocity in the line is lowered in proportion to r compared to that inair. On the other hand, it is well known that the propagation velocityof a radio-frequency transmission line whose propagation medium has aneffective relative permeability ,u' and a permittivity e' is not changedif a portion, or all, of this propagation medium is replaced by anotherpropagation medium of 3,413,575 Patented Nov. 26, 1968 relativepermeability and the effective relative dielectric constant for a givenconductor configuration and size. One such transmission line isdescribed in a co-pending Brueckmann application Ser. No. 402,669, filedOct. 8, 1964 and which issued as Patent No. 3,324,426 on June 6, 1967.

In the above noted systems, it was found that high effective relativepermeabilities pen and permittivity or dielectric constants e could beobtained by utilizing axially aligned alternate slices of ferrite andceramic of high dielectric constant in place of the ferrite alone. Suchstructures proved difficult to design and fabricate inasmuch as it wasrequired that the slices be only a friction of the operating Wavelengthin thickness. Moreover, the use of such structures was limited inasmuchas they usually required that the impedance be varied step-wiselongitudinally in order to raise the values of the effective relativepermeabilities he and dielectric constants 6 It is an object of thepresent invention to provide a transmission line wherein the above notedlimitations are overcome.

It is another object of the invention to provide a transmission linewhich is simpler to construct, has low loss, and which requires nosliding contacts.

In brief, there is provided a constant impedance, lowloW-loss delay linewhich includes a coaxial transmission line section comprising an innerconductor and an outer conductor, and a radially stratified propagationmedium intermediate the conductors which is adapted to be slideablypositioned over. the inner conductor. The propagation medium includes aplurality of ferrite-ceramic elements spaced apart by means of plasticspacers. Each of the ferrite-ceramic elements comprises a ceramic bodyand a ferrite body concentrically disposed with respect to the coaxialline conductors and disposed along a common radial plane. Theferrite-ceramic elements and plastic spacers therebetween include aradial slot and are aligned to form one integrated longitudinal slotalong the inner conductor. Included further are a plurality of spacedplastic rings, one for each ferrite-ceramic element, intermediate thepropagation medium and the outer c-oaxial line conductor and affixedthereto. Depending from each of the ceramic rings is a wedge-shapedferrite slug dimensioned so as to complement the slots of theferriteceramic elements when the propagation medium is axiallypositioned such that the ferrite-ceramic elements are in register withthe ceramic rings, and to fully complement the spacer slots when thepropagation medium is axially positioned such that the ferrite ceramicelements are out of register with the ceramic rings. The delay of an RFsignal applied to the coaxial transmission line may be varied between aminimum delay produced when the ferrite slugs are completely removedfrom the ferrite-ceramic elements, and a maximum delay when the ferriteslugs fully complement the slots of the ferrite-ceramic elements.

For a better understanding of the invention, together with other andfurther objects thereof, reference is had to the following descriptiontaken in connection with the accompanying drawing in which:

FIG. 1 is a sectional view of one element of the delay line;

FIG. 2 is a graph useful in obtaining the parameters of the sectionshown in FIG. 1;

FIG. 3 is a longitudinal cross section of a delay line according to theteachings of the invention;

FIG. 4 is an exploded view of a section of the wave propagation mediumcomprising the delay line;

FIGS. 5A and 5B illustrate cross sections of the delay line of FIG. 3taken along the lines 5-5 and represent the two extreme axial positionsof the delay line;

FIG. 6 is a schematic diagram illustrating the invention; and

FIG. 7 shows a control bar for use in the delay line.

FIG. 1 represents a cross section of a coaxial transmission line 10 ofunit length which includes a ferrite-ceramic RF wave propagation mediumcomprising the basic element or component of the present invention. Adiscussion of the underlying principles involved in the structureembodied in FIG. 1 will greatly enhance the understanding of the presentinvention. In FIG. 1, the outer conductor of coaxial line 10 is shown at12, and the inner conductor is shown at 14. concentrically arranged withrespect to the inner and outer conductors of coaxial line 10 andcoextensive therewith are a ceramic medium or body 16 and a ferritemedium or body 18, with both the ceramic medium 16 and ferrite medium 18being concentric and terminating at radial surfaces 20 and 22, as shown,to provide an axial slot 24. The concentrically arranged media 16 and 18occupy substantially all the space between outer conductors 12 and innerconductor 14, except for the slot 24, and the radial surfaces boundingslot 24 include the angle therebetween which is hereinafter referred toas the slot width. The concentrically arranged media 16 and 18 comprisesan RF propagation medium and is hereinafter referred to as theferrite-ceramic element. Although the ferrite body 18 is shown as "beingintermediate ceramic body 16 and outer conductor 12, the positionsthereof may be reversed with the ferrite body 18 placed adjacent innerconductor 14. Assuming the dimensions shown in FIG. 1, that is, nocylindrical air gaps are present, the effective relative permeabilitycan be shown to be 6 2 I In d i i 'iil (1) where #2 is the relativepermeability of the ferrite medium and the relative permeability of theceramic medium -1. The effective relative dielectric constant, orpermittivity, of the line shown in FIG. 1 can be shown to be where 6 isthe permittivity of the ceramic medium 16 and 6 is the permittivity ofthe ferrite medium 18. In FIG. 2, the ,u and a, of Equations 1 and 2 areplotted versus the normalized diameter 8/d which ranges from unity toD/d=2.3. This value of D/d was selected arbitrarily by way ofillustration and it yields a characteristic impedance of 50 ohms (K '=60ln D/d) when the propagation medium consists of air only. The relativepermeability ,LL2=40, the permittivity 9 :85, and the permittivity e=1O. If the position of the ferrite and ceramic bodies are interchanged,as mentioned above, with the ferrite body 18 placed adjacent innerconductor 14, then -1 and be adjusted since these properties are relatedto K and V where c is the velocity of light in a vacuum. Hence it can beseen that K and V are governed ultimately by Equations 4 and 5. It isevident from the examination of FIG. 2, or from Equation 1, that pstrongly depends upon the slot width, or angle while a is affected bythe angle 4 to a much smaller degree. In a first approximation, e may beregarded as independent of 5. On the other hand, ,u is not greatlyinfluenced by cylindrical air gaps, while e is severely reduced by evensmall radial clearances. Thus, for the radially-stratifiedferriteceramic media shown in FIG. 1, pm can be adjusted over a widerange by merely modifying the slot with and, on the other hand, e may beadjusted by introducing or changing the cylindrical air gaps. It hasalso been shown that am suffers little change with changes in air gaps,as long as they are kept reasonably small, and a is little affected bysmall changes in slot width or angle It is interesting to note that =e-135 where 5/D=1.3

and =10. Using the Well known fact that the propagation in the line islowered in proportion to compared to air, it can be seen that for thechoice of slot width of the propagation velocity of the line shown inFIG. 1 will be of the propagation velocity of light in vacuum.

With the above principles in mind, reference is made now to FIGS. 3-5wherein there is shown at a constant impedance, low-loss line stretcheror delay line suitable for operation at relatively high RF frequencies.The transmission line illustrated in FIGS. 35 operates on the well knownprinciple that the characteristic impedance K, of a transmission linewhose dielectric is air is unaffected if the air is replaced entirely orin part by a propagation medium whose effective relative permeabilityand dielectric constant are equal while the velocity of propagation isreduced in inverse proportion to the square root of their product(Equation 5). The line stretcher 30 includes a coaxial transmission linehaving an outer conductor 32 and an inner conductor 34 concentricallysupported at its ends within outer conductor 32 by means oflongitudinally spaced parallel-arranged radial conductors 36 and 38.Radial conductors 36 and 38 extend through and are affixed to insulatorplugs 40 and 42 respectively provided therefor. As shown, insulatorplugs 40 and 42 are affixed to counter conductor 32 to form an integralpart thereof. If desired, input conductor 36 and output conductor 38 canalso be concentrically positioned with inner conductor 34 at the endsthereof.

Slideably mounted on inner conductor 34 and substantially coextensivetherewith is an RF propagation medium 46 which is shown in detail inFIG. 4. Referring now to FIGS. 3 and 4, the propagation medium 46comprises a plurality of axially spaced, radially-Stratifiedferrite-ceramic elements 48 which are constructed in accordance withtheprinciples described above in connection with FIG. 1. As shown, each ofthe elements 48 comprises a ceramic body or medium 50 and a ferrite bodyof medium 52 concentrically arranged about inner conductor 34, and anaxial slot 54 bounded by radial surfaces 56 and 58 as shown in FIG. 5B.The ferrite body 52 may be made of any of the well known commercialferrite materials adapted for use at radio-frequencies such as thoseknown as Q or Q or Q Also, the ceramic body 50 may comprise any suitablematerial such as titanium dioxide (TiO which has a high dielectricconstant and low loss, and with a permeability -1. Ferrite body 52 isafiixed to ceramic body 50 and the axial thickness, t, of each of theelements 48 is made about 3 of the wavelength in the ferrite-ceramicelements 48 at the highest operating frequency. The axial slots 54 ofthe spaced ferrite-ceramic elements 48 are of same slot width, or angleand are axially aligned along the inner conductor 34. The ferriteceramicelements 48 are axially spaced from each other by the axial dimension t,and the spacing between elements 48 is maintained by means of plasticspacers 60 cemented to the elements 48. Such plastic spacers may be madeof a material such as Teflon, or foam polystyrene, having values ofpermeability and dielectric constant very close to that of air, that is#:ezl. As shown, each of the plastic spacers 60 is provided with anaxial slot 54' having the same dimensions as axial slot 54 of theelements 48, and all the slots are aligned to form a continuouslongitudinal slot 61 along the inner conductor 34. The continuouslongitudinal slot 61 is aligned with the radial conductors 36 and 38which are made narrower in width than the longitudinal slot 61. The highfrequency RF input signals to be delayed may be applied to the coaxialsection through radially or axially disposed conductor 36 and thedelayed output signal may be derived from radially or axially disposedconductor 38. The radial dimensions of the ferrite-ceramic elements 48is readily determined once the slot width dimension is chosen asexplained above in connection with FIGS. 1 and 2. The central bore 62 ofpropagation medium 46 comprising the identically constructedferrite-ceramic elements 48 and identically constructed spacers 60 ismade to fit as tightly as possible around inner conductor 34 and yetpermit the propagating medium 46 to be slideably positioned axiallyalong the inner conductor 34 for the distance r. Of course, any suitablemechanism well known in the art may be utilized to limit thelongitudinal movement of propagation medium 46 to the distance 1.

Afiixed to the inner periphery of outer conductor 32 are a plurality ofaxially spaced ceramic rings 64 each having the axial dimension t, andspaced from one another by the same axial dimension, 2. As explainedabove, the axial dimension, t, is preferably chosen to be about of thewavelength at the highest operating frequency and is identical to thatof the ferrite-ceramic elements 48. As

shown, the ceramic rings 64 are substantially coextensive with innerconductor 34 and a respective ceramic ring 64 is provided for each ofthe ferrite-ceramic elements 48 comprising propagation medium 46.Depending from the inner periphery of each ceramic ring 64 is awedge-shaped ferrite slug 66 so dimensioned that it is adapted to fullycomplement the axial slot 54 when a respective ceramic ring 64 isaxially aligned with its corresponding ferriteceramic element 48. Likethe respective axial slots 54 and 54 of the ferrite-ceramic elements 48and spacers 60 comprising propagation medium 46, the wedge-shapedferrite slugs 66 are longitudinally aligned so as to be in register withthe integrated longitudinal slot 61 of the propagation medium 46 alongthe inner conductor 34. The ceramic rings 64 are positioned such thatwhen the propagation medium 46 is in one extreme position, respectiveferriteceramic elements 48 of propagation medium 46 are in register withassociated ceramic rings 64 and when propagation medium 46 is in theother extreme position, the plastic spacers 60 of propagation medium 46are in register with the ceramic rings 64. It can be seen that when therespective ceramic rings 64 are in register with associatedferrite-ceramic elements 48, the respective ferrite slugs 66 fullycomplement the axial slots 54 of their associated ferrite-ecramicelements 48 and at the same time the respective ceramic rings 64completely surround their associated ferrite-ceramic element. With suchan arrangement, the cylindrical air gaps between the ferrite-ceramicelements 48 and outer conductor 12 of coaxial line 10 are diminished andthe slots 54 do not exist. This is shown in FIG. 5A. On the other hand,when the respective ceramic rings 64 are in register with the associatedplastic spacers 60 of propagation medium 46, the respective ferriteslugs 66 complement the axial slots 54' of their associated plasticspacers. In this position, a maximum cylindrical air gap exists betweeneach ferrite-ceramic element 48 and outer conductor 12 together with theaxial slots 54. This is shown in FIG. 5B. It is to be understood ofcourse, that there is just enough tolerance or clearance between theinner periphery of the ceramic rings 64 and the outer periphery ofpropagation medium 46 to permit relative axial movement therebetween.

In operation, when the ceramic rings 64 are in register with theferrite-ceramic elements 48 the diameter 6 of ceramic body 50 may bechosen so that =e is obtained. These will be high in value. When theceramic rings 64 and the ferrite-ceramic elements are disengaged by anaxial displacement, both a cylindrical air gap and the slot now exist.The slot width 15 may be chosen so that am is quite low. When is thuschosen the outer diameter D (FIG. 5B) of the ferrite body 52 for ,u.=eis automatically specified. This assures that K=K for each cross sectionof the transmission line. As a numerical example, a line stretcher ordelay line has been designed whose n =e -l9 when the ferrite-ceramicelements 48 of propagation medium 46 and the ceramic rings 64 areengaged in or register, and when not engaged,

FIG. 6 shows schematically how the line stretcher functions. In region1a the medium has low effective relative values u =e by virtue of theslot and air gaps. In region 1b the medium has high efiective values u=e because of the diminished or negligible slot and negligiblecylindrical air gap. Finally, in region 2 the effective relative valuesare nearly unity by proper design.

The effective relative values of this stacked arrangement are,therefore,

In one embodiment the application of Equation 5 showed that Thisresulted in a constant impedance line stretcher whose electrical lengthor delay can be varied over a 2.7 to 1 range with a maximum electricallength of about 10 times its physical length. It is interesting to notethat inasmuch as this range of adjustment is accomplished by sliding theinner medium through the small distance t, the axial thickness of asingle ferrite-ceramic element, the delay line in accordance with thepresent invention will be much shorter than other delay lines such asthat shown in co-pending Brueckmann application Ser. No. 315,093. Atraverse control, such as a screw thread mechanism with a calibrateddial may be utilized in a conventional manner to adjust the delay.

Although the slot width of the propagation medium 46 is shown to beangular, that is, bounded by radially disposed surfaces, it need not beso limited. A slot of uniform width, for example, may be utilized andthe relationship of values of am and e to the slot width can beestablished empirically or theoretically.

In FIG. 3, the ,u and 6 are controlled by axially shifting thepropagation medium 46 while the ceramic rings are fixed in position.This same result can be achieved by making the slugs 66 of ceramic ring64 axially shiftable and maintain in fixed position both the ceramicrings 64 and the propagation medium 46. This can be achieved byutilizing a control bar 70 as Shown in FIG. 7. The control bar 70 iswedge-shaped in cross section and is dimensioned to fully complementlongitudinal slot 61 both in length and depth and to completely occupythe space between the ceramic rings 64 and propagation medium 46. Asshown, control bar 70 is comprised of alternate slices of a ferrite body72 and a plastic body 74 each having the axial dimension t. The ferritebody 72 and plastic body 74 are of the same material, respectively, asthat of the ferrite in ferriteceramic element 48 and the plastic spacers60. Any suitable means may be utilized to axially shift control bar 70through the axial distance 2. The resulting line has controllableimpedance and controllable electrical length as hereinabove described.

While the delay line of FIGS. 3-5 shows the propagation medium 46 tocomprise ferrite-ceramic elements as .shown in FIG. 1, it is to beunderstood of course, that the ferrite body 52 and ceramic body 50 maybe interchanged in the ferrite-ceramic elements 48 without effectivelychanging the operation of the line 30. In this connection, the dimensionD (FIG. 5B) may be derived in accordance with the following equation:

where 6 is that shown in FIG. 6. As hereinabove noted, D is the same asthe inside diameter of ceramic rings 64 except for a small negligibleclearance.

While there has been described what is at present considered to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is therefore aimedin the appended claims to cover all such changes and modifications asfall within the true spirit and scope of the invention. For example, thebasic principle of radial stratification may be applied as Well to twowire lines, strip lines, tapered lines, etc.

What is claimed is:

1. A variable low-loss delay line comprising a coaxial transmission linehaving an outer conductor and an inner conductor, an RF propagationmedium concentric With said conductors and comprising a plurality ofaxially spaced radially stratified ferrite-ceramic elements, each ofsaid elements including a slot bounded by radial surfaces, said slotsbeing axially aligned, said propagation medium being adapted to beaxially positioned along said inner conductor, and means intermediatesaid outer conductor and said propagation medium operatively associatedwith said propagation medium whereby when said propagation medium is atone extreme axial position, a cylindrical air gap exists between each ofsaid ferriteceramic elements and said outer conductor, and respectiveferrite-ceramic element slot gaps exist between said conductors, andwhen said propagation medium is in the opposite extreme axial position,each of the radially stratified ferrite-ceramic elements extend as asolid body from said inner conductor to said outer conductor.

2. The low-loss delay line in accordance with claim 1, wherein theferrite body of each of said elements re spectively encompasses andabuts associated ceramic bodies.

3. The low-loss delay line in accordance with claim 1 wherein theceramic body of each of said elements respectively encompasses and abutsassociated ferrite bodies.

4. A variable low-loss delay line comprising a coaxial transmission linehaving an outer conductor and an inner conductor, an RF propagationmedium concentric with said conductors and comprising a plurality ofspaced ferrite-ceramic elements, each of said elements comprising aceramic body and a ferrite body concentrically arranged with saidconductors along a common radial plane to effect a radially stratifiedstructure between said conductors, said ferrite body encompassing andabutting said ceramic body, said ferrite-ceramic elements each includinga slot bounded by radial surfaces extending from said inner conductor,said slots being longitudinally aligned, and a plurality oflongitudinally spaced ferrite slugs extending from said outer conductortowards said inner conductor and dimensioned so as to fully complementsaid slots when in register therewith, said propagation medium beingadapted to move axially along said inner conductor such that respectiveslots are in register with associated ferrite slugs when saidpropagation medium is in one extreme axial position, and respectiveslots are out of register with said ferrite slugs when said medium is inan opposite extreme axial position.

5. The delay line in accordance with claim 4 wherein saidferrite-ceramic elements are separated by plastic spacers having apermeability and a dielectric constant substantially equal to unity.

6. A variable low-loss delay line comprising a coaxial transmission linehaving an outer conductor and an inner conductor, an RF propagationmedium concentric with said conductors and comprising a plurality ofradially stratified ferrite-ceramic elements axially separated byplastic spacers, each of said ferrite-ceramic elements comprising aceramic body proximal said inner conductor, and a ferrite body affixedto said ceramic body, said propagation medium being adapted to beaxially positioned along said inner conductor, each of saidferriteceramic elements and said spacers having axially aligned slotsbounded by radial surfaces extending from said inner conductor to forman integrated longitudinal slot along said inner conductor, a pluralityof spaced ceramic rings, one for each of said ferrite-ceramic elements,affixed to the inner periphery of said outer conductor, respectiveferrite slugs depending from respective ceramic rings and longitudinallyaligned with said longitudinal slot, said rings and said slugs being sodimensioned such that when said ferrite-ceramic elements and saidceramic rings are in register, respective slugs complement associatedslots of said ferrite-ceramic elements and respective ceramic ringsencompass associated ferrite-ceramic elements to form a solid bodybetween said conductors, and when said ceramic rings and saidferrite-ceramic elements are out of register, a cylindrical air gapexists between each of said ferrite-ceramic elements and said outerconductor and respective ferrite-ceramic element slot gaps exist betweensaid conductors.

7. The low-loss line in accordance with claim 6 wherein saidferrite-ceramic elements have an axial dimension, t, the spacing betweensaid ferrite-ceramic elements being said dimension t, and the axialdimension of said rings is said dimension 1.

8. A variable low-loss delay line comprising a coaxial transmission linehaving an outer conductor and an inner conductor, an RF propagationmedium concentric with said conductor and comprising a plurality ofaxially spaced ferrite-ceramic elements, each of siad elementscomprising a ceramic body encompassed by a ferrite body concentricallyarranged with said conductors along a common radial plane to effect aradially stratified structure between said conductors, saidferrite-ceramic elements each including a slot bounded by radialsurfaces extending from said inner conductor, said slots beinglongitudinally aligned, a plurality of longitudinally spaced ceramicrings, one for each of said ferrite-ceramic elements, aflixed to saidouter conductor with said coaxial line, respective rings being inregister with respective ferrite-ceramic elements, and an axially moveable bar dimensioned to fully complement said longitudinally aligned slotand disposed intermediate said longitudinal slot and said ceramic rings,said bar comprising spaced sections of ferrite separated by plasticspacers, the elfective relative permeability and the permittivity ofsaid propagation medium being determined by the axial distance the barferrite sections are within each of said ferrite-ceramic slots.

References Cited UNITED STATES PATENTS 2,228,798 1/1941 Wasserman 52,877,433 10/1959 Deyot 333-73 3,219,950 11/1965 Brueckmann 333313,274,521 9/1966 Nourse 333 -1.1 3,275,954 9/1966 Coda et a1. 333-793,329,911 7/1967 Schlicke et a1. 333-79 10 HERMAN KARL SAALBACH, PrimaryExaminer.

C. BARAFF, Assistant Examiner.

